1) Field of the Invention
The present invention relates to a lithium nickel composite oxide serving as a positive electrode active material and a method for judging the quality of a positive electrode using this substance.
2) Description of the Related Art
Non-aqueous electrolyte secondary cells, for their high energy density and high capacity, are widely used as power sources for mobile appliances. Conventionally, as the positive electrode active material used for the non-aqueous electrolyte secondary cells, lithium cobalt composite oxide (LiCoO2), which excels in discharge property, has been used.
However, an increasing need for further enhancement of cell capacity and an increase in cell cost due to rising prices of cobalt have drawn attention to lithium nickel composite oxide LiaNixM1-xO2 (where M is at least one selected from Co, Al, Zr, Ti, Mg, and Mn, 0.9≦a≦1.1, and 0.5≦x≦1) as the positive electrode active material of non-aqueous electrolyte secondary cells.
However, the lithium nickel composite oxide still possess problems to be solved, and cells using the lithium nickel composite oxide have a higher occurrence rate of defectiveness than cells using lithium cobalt composite oxide.
As an example of the problems to be solved, non-patent document 1 (“Abstracts of Speeches at the 47th Battery Symposium,” pp. 326-327) reports that if a lithium nickel composite oxide exposed to atmosphere is used to constitute a non-aqueous electrolyte secondary cell, cell swelling occurs from high-temperature preservation.
Non-patent document 1 gives a possible cause for such cell swelling as follows. If the lithium nickel composite oxide is exposed to atmosphere, the lithium ions in the lithium nickel composite oxide react with moisture in atmosphere whereby a highly reactive lithium hydroxide occurs, which in turn reacts with carbon dioxide in atmosphere to result in lithium carbonate (Li2CO3). Also, the moisture contained in the atmosphere-exposed lithium nickel composite oxide decomposes LiPF6 serving as electrolytic salt inside the cell to generate hydrofluoric acid (HF). This in turn decomposes the lithium carbonate (Li2CO3), thereby generating carbon dioxide gas inside the cell. The gas generated inside the cell possibly remains between the positive and the negative electrodes to the detriment of the opposing condition thereof, resulting in a decrease in cell capacity.
The generation of lithium hydroxide possibly causes a decrease in cell capacity also in such a respect that the amount of lithium nickel composite oxide that contributes to charge/discharge decreases.
Incidentally, as a method for solving these problems associated with the lithium nickel composite oxide, carrying out the whole process of cell production under conditions without exposure to atmosphere, such as in a dry air atmosphere and an inactive gas atmosphere, are contemplated. However, this method causes a significant increase in production cost. Therefore, this method is not practical.
In view of these circumstances, such a method is conventionally employed that tests for cell swelling, a decrease in cell capacity, and the like are carried out after a cell is complete, and when the cell is judged to be unsuitable, all the cells of the same production lot are discarded. However, this method may significantly degrade the production yield, and the decreased production yield causes the problem of raising the price of the complete cell.